1. Technical Field of the Invention
The present invention relates generally to a gas concentration measuring apparatus for measuring the concentration of gases which may be employed in an air-fuel ratio control system for automotive vehicles, and more particularly to a gas concentration measuring apparatus designed to minimize an error component contained in an output thereof.
2. Background Art
Recently, NOx sensors designed to measure the concentration of nitrogen oxide (NOx) contained in exhaust emissions of automotive engines are proposed and put into practical use.
As one of such NOx sensors, a gas sensor is known which is designed to measure the concentrations of NOx and O2 contained in exhaust gasses of the engine simultaneously. This type of gas sensor includes a pump cell for decomposing or ionizing oxygen molecules contained in exhaust gasses to measure the concentration of O2 and a sensor cell for decomposing NOx in the oxygen-decomposed exhaust gasses to measure the concentration of NOx. The measurement of the concentration of each of NOx and O2 is achieved by applying a given voltage to a corresponding one of the pump cell and the sensor cell to induce flow of current as a function of one of the concentrations of NOx and O2. The current is outputted from the gas sensor and converted into a voltage signal which is, in turn, used in, for example, an engine control unit of the vehicle.
The above gas sensor, however, has the drawback in that the amount of current flowing through the cell sensor as a function of the concentration of NOx is extremely small, so that it apt to interfere with electrical noises, resulting in a failure in measuring the concentration of NOx accurately. Specifically, when the concentration of NOx is within 0 to 2000 ppm, a current output from the sensor cell is as little as 5 to 10 μA. Therefore, in the case where the gas sensor is used in an engine control system of an automotive vehicle, signal outputs from peripheral electrical devices are added to an output of the gas sensor as noises which will produce an error in measuring the concentration of NOx.